1. Field of the Invention
The present invention relates to an inter-device or intra-device data communication system where data are exchanged between a master station and a plurality of slave stations.
2. Description of the Related Art
FIG. 9 shows a conventional IOM-2 system (ISDN Oriented Modular Interface) based point-multipoint data communication system having a master station and a plurality of slave stations. The IOM-2 system is detailed in "ICs for Communications--IOM-2 Interface Reference Guide" published by semiconductor group, Siemens Aktiengesellschaft, Ordering No. B115-B6161-X-X-7600 Printed in Germany TS 01905. IOM-2 is a registered trademark of Siemens AG.
Referring to the figure, a master station 19 has a plurality of internal master communication devices 1-1 through 1-8, each of which corresponds to one of slave communication devices 2-1 through 2-8 in slave stations. The respective slave stations are coupled to the master station via communication channels of a Frame Synchronization Clock (FSC) line 3, a Data Clock (DCL) line 4, a Data Downstream (DD) line 5 with a pull-up resister 8 and a Data Upstream (DU) line 6 with a pull-up resister 9. The respective communication channels couple one of the internal master communication devices 1-1 through 1-8 to a corresponding one of the slave communication devices 2-1 through 2-8. The master station is also connected with a microprocessor 7.
FIG. 10 shows a timing diagram illustrating conventional transmission frame formats of data based on IOM-2 system.
Referring to the figure, a frame format 10 depicts a typical signal format in a signal subchannel among a plurality of signal subchannels IOM CH0 through IOM CH7. The frame format 10 is a common frame format to both upward and downward data transmitted via the Data Upstream DU and Data Downstream DD lines.
The signal frame format 10 is composed of two eight-bit B-channels B1 and B2 for user data, an eight-bit Monitor for message elements, a six-bit C/I and two one-bits of MR and MX for other data. C/I indicates an internal/external status of a master/slave communication device and indicates an instruction to the other side of the master-slave communication device of an operation. A total capacity of the signal frame format 10 is 32 bits, and therefore, that of a frame with eight signal subchannels of the eight slave stations is 256 bits, the calculation of which is as follows: EQU 8 bits.times.4 data fields.times.8 subchannels=256 bits
According to the IOM-2 system based conventional point-multipoint data communication system, the master station 19 outputs an 8 (eight) KHz single-cyclic frame synchronization clock signal to the Frame Synchronization Clock (FSC) line 3 and a 4.096 MHz data clock signal to the Data Clock (DCL) line 4. The master station exchanges data privately with the respective slave stations via corresponding signal subchannels on the Data Upstream and Downstream lines 5 and 6. The signal subchannels IOM CH0 through IOM CH7, each of which has an eighth time period of the single-cyclic frame synchronization clock signal on the Frame Synchronization Clock (FSC) line 3, are provided for transmitting data privately between the respective internal master communication devices 1-1 through 1-8 and corresponding slave communication devices 2-1 through 2-8.
Thus, in this conventional system, a transmission frame of 256 bits is exchanged between the master station and the slave stations in the point-multipoint communication system. In this case, a transmission speed of 2 Mbps is required in a data transmission system, as the following calculation shows. EQU 256 bits+125 .mu.S=2 Mbps
That transmission speed, however, would cause a transmission overload problem in such a situation as a plurality of slave stations operating with the maximum working capacity are trying to communicate with a single master station in the conventional system. A solution to this problem is, therefore, that a plurality of internal communication circuits must be added to the master station so that the number of the internal communication circuits may even the number of slave stations for a normal communication. This solution, however, leaves a problem of higher production cost and larger scale or space of a data transmission system.
The present invention is to solve this and other problems. An object of the invention is to provide a point-multipoint data communication system to achieve an effective data communication between a master station having a single internal communication circuit and a plurality of slave communication circuits by way of the smallest number of signal channels required in the system.